JPH03203454A - Linear image sensor - Google Patents

Abstract

PURPOSE:To reduce the generation of a fixed pattern noise due to the ununiformity of amplifier FET threshold voltage and mutual conductance by subtracting a dark signal output line signal from a signal obtained by delaying a light signal output line signal only by one clock. CONSTITUTION:A light output signal from a photodiode 1a is led into a light signal output line by a scanning signal Y1 and its dark output signal is led into a dark signal output line by a scanning signal Y2. Similarly a light output signal of a photodiode 2a is led into the light signal output line by the signal Y2 and its dark output signal is led into the dark signal output line by a scanning signal Y3. Since the dark output signal of each picture element is outputted with the delay of one clock period from the light output signal, the fixed pattern noise generated due to the ununiformity of the amplifying FET at the dark time can be removed by subtracting the dark output signal from the output signal on the same time axis after delaying the light output signal by one clock period.

Description

DETAILED DESCRIPTION OF THE INVENTION With the advancement of information processing equipment in industrial applications, the importance of image sensors as input devices has increased. The present invention relates to an image sensor that makes it possible to read document information with high sensitivity, high speed, and high S/N.

Conventional technology An example of an image sensor formed on a Si crystal using integrated circuit technology is a CCD image sensor.

There are MOS image sensors, bipolar IC image sensors, etc.

A CCD image sensor uses a photodiode as a light detection element and a CCD (charge-coupled device) as a scanning circuit. After accumulating the optical signal charge generated by the photodiode for a certain period of time, it is transferred to the CCD and transferred sequentially. An image signal is obtained by amplifying and converting the signal charge into a voltage using an amplifier built into the Si chip.

A MOS image sensor uses a photodiode as a light detection element and a MOS shift register as a scanning circuit, and generates an image signal by sequentially recharging the discharged charge caused by the photocurrent in the photodiode according to a shift signal from the shift register. It has gained. In this method, since the discharge charge in the photodiode is minute, the obtained image signal is small and the S/N is low. Nowadays, rather than extracting the photodiode's discharged charge directly, an image signal is obtained by amplifying the change in the photodiode's terminal voltage due to incident light using an FET (field effect transistor) attached to each photodiode. A sensor, namely an amplified MOS image sensor, is currently under development.

A bipolar IC image sensor uses a phototransistor as a light detection element and a bipolar IC shift register or decoder as a scanning circuit.The bipolar IC image sensor uses a photocurrent to discharge charges stored in the parasitic capacitance between the base and collector of the phototransistor. By sequentially recharging the battery according to the access from the scanning circuit, an image signal is obtained through the amplification function of the phototransistor.

Problems to be Solved by the Invention In an amplified MOS image sensor, an amplifying FET is attached to one end of each photodiode, and fixed pattern noise increases due to non-uniformity of the FET's threshold voltage VT and mutual conductance gm. do. An increase in fixed pattern noise degrades document reading quality.

Means to solve the problem Each pixel is equipped with a photodiode, an amplification FET, an access FET, a reset FET, and a dark signal access FET.
Create a configuration with ET. One end of the photodiode is connected to the gate of the amplification FET and the drain of the reset FET. The signal access FET becomes conductive during access and guides the optical signal from the amplification FET to the optical signal output line. The dark signal access FET becomes conductive upon reset and guides the dark signal from the amplification FET to the dark signal output line.

Note that the reset FET sets the initial charging voltage of the photodiode.

The optical signal of each working pixel appears in sequence on the optical signal output line in accordance with the shift signal, and the dark signal of each pixel appears in sequence in the dark signal output line in accordance with the shift signal with a delay of one clock period. By subtracting the signal on the dark signal output line from the signal obtained by delaying the signal on the optical signal output line by one clock cycle, fixed pattern noise due to non-uniformity of VT and gm of the amplification FET can be reduced.

Example Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is an equivalent circuit diagram of an image sensor according to a first embodiment of the present invention. In the figure, photodiode l
The cathodes of a, 2a, and 4a are connected to the positive power supply as a common terminal, and the anodes of photodiodes la, 2a, and 4a are connected to the gates of amplification FETs 1b, 2b, and 4b, and the drains of reset FETs 1e, 2e, and 4e, respectively. Connected. The sources of the amplification FETs 1b, 2b, and 4b are the optical signal access FETs 1c and 2c, respectively.

4c drain and dark signal access FETs 1d, 2
Connected to the drains of d and 4d. A shift register 6 is formed of a MOS-IC and is driven by an external start signal and clock signal. The n-dot image sensor uses an n+1 stage shift register,
If, 2f.

3f, 4f, and 5f are parallel output terminals of the shift register, which are sequentially connected to gate electrodes of access and reset FETs. Therefore, the dark signal access FET is connected so as to operate one clock cycle later than the optical signal access FET. FET1c for optical signal access
, 2c, and 4c are commonly connected to form an optical signal output line 7, and a dark signal access FET1d,
The sources of 2d and 4d are commonly connected to form a dark signal output line 8. In addition, reset FET1e, 2e
, 4e are commonly connected to serve as an input terminal for the reset voltage VH8.

Next, the operation of this image sensor will be explained. FIG. 2 is an operation timing chart of this image sensor, in which the clock signal CK, start signal ST, and parallel output signals Y1, . Y2゜Y3, Yn
, Y n + l and the light output signal and dark output signal are shown.

At the time of reset, this sensor stores a certain amount of charge by charging the photodiode with a certain voltage (Vdd-Vrs), and then amplifies the change in the terminal pressure of the photodiode due to the discharge of the charged charge by the photocurrent. For F
The amplified optical output signal 4- is received by the gate of the ET. In particular, non-uniformity in the characteristics of the FET causes non-uniformity in signal current, causing fixed pattern noise. Since there is no discharge due to photocurrent in the dark, the terminal voltage of the photodiode is held at the reset voltage until the next reading timing. In other words, the fixed pattern noise signal in the dark is equal to the output signal obtained at the timing when the reset FET becomes conductive. In FIG. 2, the optical output signal of the photodiode 1a is guided to the signal output line by the scanning signal Y1, and its dark output signal is guided to the dark signal output line by the scanning signal Y2. Similarly, the optical output signal of the photodiode 2a is guided to the optical signal output line by the scanning signal Y2, and its dark output signal is guided to the dark signal output line by the scanning signal Y3. Since the dark output signal of each pixel is output one clock cycle later than the optical output signal,
By delaying the optical output signal by one clock period and then subtracting the dark output signal from the output signal on the same time axis, it is possible to remove the fixed pattern noise during the dark period due to the non-uniformity of the amplification FET.

FIG. 3 is an equivalent circuit diagram of an image sensor according to a second embodiment of the present invention.

The sensor of this embodiment exhibits a negative characteristic in which the output signal decreases in proportion to the amount of light due to the difference in the polarity of the photodiode. However, as in the first embodiment, by subtracting the dark output signal from the light output signal, , it is possible to remove fixed pattern noise in the dark.

FIG. 4 is a circuit diagram in which the optical output signal is delayed by one clock cycle and differentially amplified with the dark output signal.

1/'V is a current-voltage converter, φ1. φ2 is a two-phase clock pulse, C1 and C2 are hold capacitors, and VO is an image output signal. Note that C1>>C2. The capacitor and analog switch function to delay the image output signal by one clock, and the differential amplifier reduces fixed pattern noise by differentially amplifying the delayed image signal and the dark output signal.

Effects of the Invention According to the present invention, fixed pattern noise of an amplified MOS image sensor can be reduced, and a document can be read with high quality. Therefore, this image sensor is extremely useful as an input device for information processing equipment.
Its industrial effects are significant.

[Brief explanation of drawings]

FIG. 1 is an equivalent circuit diagram of an image sensor according to the first embodiment of the present invention, FIG. 2 is an operation timing chart, and FIG.
FIG. 4 is an equivalent circuit diagram of an image sensor according to a second embodiment of the present invention, and FIG. 4 is a block diagram of an embodiment of a delay circuit and a differential amplifier connected to the image sensor according to the embodiment of the present invention. la, 2a, 4a...photodiode, lb
. 2b, 4b...Amplification FET, lc, 2c, 4
c...FET for optical signal access, ld, 2d,
4d...FET for dark signal access, 7...
...Light signal current output terminal, 8...Dark signal current output terminal.

Claims (3)

[Claims] (1) In an amplification MOS image sensor in which each pixel consists of a photodiode, an amplification field effect transistor (FET), an access FET, etc., a dark signal is used to output the source current of the amplification FET when the photodiode is reset. A linear image sensor characterized by being equipped with an access FET. (2) Each access FET is driven by an access signal, its drain is connected to the source of each amplification FET, the source is commonly connected between pixels to form an optical signal output line, and each dark signal access FET is connected to the source of each amplification FET. is driven by a reset signal, its drain is connected to the source of each amplification FET, and its sources are commonly connected between pixels to form a dark signal output line.
Linear image sensor described in section. (3) The fixed pattern noise is reduced by differentially amplifying the signal obtained by delaying the signal of the optical signal output line by one clock period and the signal of the dark signal output line. Linear image sensor.